Search for single production of vector-like quarks decaying into $Wb$ in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

A search for singly produced vector-like quarks $Q$, where $Q$ can be either a $T$ quark with charge $+2/3$ or a $Y$ quark with charge $-4/3$, is performed in proton-proton collision data at a centre-of-mass energy of 13 TeV corresponding to an integrated luminosity of $36.1 \text{fb}^{-1}$, recorded with the ATLAS detector at the LHC in 2015 and 2016. The analysis targets $Q \to Wb$ decays where the $W$ boson decays leptonically. No significant deviation from the expected Standard Model background is observed. Upper limits are set on the $QWb$ coupling strength and the mixing between the Standard Model sector and a singlet $T$ quark or a $Y$ quark from a $(B,Y)$ doublet or a $(T,B,Y)$ triplet, taking into account the interference effects with the Standard Model background. The upper limits set on the mixing angle are as small as $|\sin{\theta_{\text{L}}}|= 0.18$ for a singlet $T$ quark of mass 800 GeV, $|\sin{\theta_{\text{R}}}|= 0.17$ for a $Y$ quark of mass 800 GeV in a $(B,Y)$ doublet model and $|\sin{\theta_{\text{L}}}|= 0.16$ for a $Y$ quark of mass 800 GeV in a $(T,B,Y)$ triplet model. Within a $(B,Y)$ doublet model, the limits set on the mixing parameter $|\sin{\theta_{\text{R}}}|$ are comparable with the exclusion limits from electroweak precision observables in the mass range between about 900 GeV and 1250 GeV.

18 December 2018

Contact: Exotics conveners internal

JHEP 05 (2019) 164

e-print arXiv:1812.07343 - pdf from arXiv

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Figures | Tables | Auxiliary Material

Figures

Figure 01

Leading-order Feynman diagram for single Y/T production in Wb fusion and subsequent decay into Wb. The production amplitude scales with sin θ_{L, R} or c_L,R^Wb as described in the text.

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Figure 02

The generated mass distributions at particle level for a Y quark with a mass of 900 GeV, for a coupling strength of c₀ ≈ 0.5 and c_L^Wb = c_R^Wb (√(c_L^Wb)² + (c_R^Wb)² ≈ 1/√2, solid line) and of c₀ = c_L^Wb = 0.14 (dotted line) as defined in Ref. 95. The dashed line shows the generated vector-like quark mass distribution at particle level of a left-handed Y signal with a mass of 900 GeV, coupling strength of c_L^Wb = 0.14 and interference effects with the SM included. The interference effects lead to negative entries in some bins of the distribution. For better visualisation of the tail distribution including the interference effect, the bin contents of all distributions were shifted by +0.1 before normalisation.

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Figure 03a

Distribution of VLQ candidate mass, m_VLQ, in the SR for three different signal masses (a) without and (b) with interference effects, for a left-handed Y signal with a mass of 900 GeV (dashed line), 1200 GeV (dotted) and 1600 GeV (dash-dotted line) and a coupling of c_L^Wb ≈ 0.29, ≈ 0.33 and ≈ 0.91 respectively, together with the total SM background (solid line). The error bars represent the statistical uncertainties. The signal events are scaled by a factor of five. Depending on the coupling and signal mass it is possible that negative entries occur in some bins of the signal-plus-interference m_VLQ distribution due to the interference effect. The distributions for a right-handed and left-handed Y signal without considering any interference effects are the same.

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Figure 03b

Figure 04

Observed background yields in the SR and in the two CRs after the fit to the data in the control regions and the signal region under the background-only hypothesis. The lower panel shows the ratio of data to the fitted background yields. The error bars, being smaller than the size of the data points and hence not visible in the top part of the plot, represent the statistical uncertainty in the data. The band represents the total (statistical and systematic) uncertainty after the maximum-likelihood fit.

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Figure 05a

Distribution of the VLQ candidate mass, m_VLQ, in (a) the SR, (b) the W+jets CR, and (c) the tt̄ CR, after the fit to the background-only hypothesis. The first and last bin include the underflow and overflow respectively. The lower panels show the ratios of data to the fitted background yields. The error bars represent the statistical uncertainty in the data. The band represents the total systematic uncertainty after the maximum-likelihood fit. An example distribution for a Y signal with a coupling of √(c_L^Wb)² + (c_R^Wb)² ≈ 0.5 without considering any interference effects is overlaid; for better visibility, it is multiplied by a factor of 30 in the W+jets CR and by a factor of 10 in the tt̄ CR. While the total uncertainty decreases when performing the fit, the total uncertainty in the bins around 1450-1600 GeV and 1850-2200 GeV in (b) does not decrease due to significant statistical MC uncertainties in these two bins.

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Figure 05b

Figure 05c

Figure 06a

Distributions of the VLQ candidate mass, m_VLQ, after the fit to the background-only hypotheses for four different binnings chosen for four different signal masses. The first and last bin include the underflow and overflow respectively. The VLQ candidate mass distributions for (a) a left-handed Y signal with mass 900 GeV and coupling c_L^Wb = 0.27, (b) a left-handed Y signal with mass 1500 GeV and coupling c_L^Wb = 0.64, (c) a left-handed T signal with mass of 800 GeV and coupling c_L^Wb = 0.25 and (d) a left-handed T signal with mass 1200~GeV and coupling c_L^Wb = 0.49 are also shown; all signal distributions include interference. The lower panels show the ratio of data to the fitted background yields. The error bars represent the statistical uncertainty in the data. The band represents the total systematic uncertainty after the maximum-likelihood fit.

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Figure 06b

Figure 06c

Figure 06d

Figure 07a

Observed (solid line) and expected (short-dashed line) 95% CL limits on (a) the mixing angle |sinθ_L| and the coupling value c_L^Wb for a singlet T-quark model assuming B(T → Wb) ≈ 0.5, (b) |sinθ_L| and c_L^Wb for a (T,B,Y) triplet model, and (c) |sinθ_R| and c_R^Wb for a (B,Y) doublet model assuming a branching ratio B(Y → Wb) = 1, as a function of the VLQ mass. The surrounding bands correspond to ±1 and ±2 standard deviations around the expected limit. The excluded region is given by the area above the solid line. Constraints from electroweak precision observables, which are only valid for the mixing angles, from either the S and T parameters (dashed-dotted line) or from the R_b values (long-dashed line), are also shown. These constraints are taken from Ref. 1, where they are presented as a function of m_B (in the (B,Y) doublet case), respectively, m_T (in the (T,B,Y) triplet case) and translated to m_Y using the value of the corresponding mixing angle constraint.

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Figure 07b

Figure 07c

Figure 08

Observed (solid line) and expected (short-dashed line) 95% CL limits on cross-section times branching ratio for the case of the right-handed Y quark for a (B,Y) doublet model as a function of VLQ mass. For the theoretical prediction, the branching ratio B(Y → Wb) is set to one. The theoretical NLO cross-sections for different coupling values are shown for the calculation using the narrow-width approximation (dashed-dotted lines) and using no narrow-width approximation (solid lines) as described in the text.

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Tables

Table 01

Generators used to model the signals and different background processes. The parameter tune for the underlying event, PDF set, and the highest-order perturbative QCD (pQCD) accuracy used for the normalisation of each sample is given. All processes, except for Yqb signals, were generated at NLO in QCD. The LO cross-sections calculated for the Yqb signal processes in the simulation were normalised to the NLO theoretical cross-section taken from Ref. [14].

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Table 02

Summary of common preselection requirements and selection requirements for the SR compared to those for the tt̄ and W+jets CRs. All other selection requirements are the same for all three regions.

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Table 03

Systematic uncertainties considered in this analysis. An uncertainty that affects normalisation only (cross-section only) for all processes and channels is denoted by ``N", whereas ``SN" means that the uncertainty affects both shape and normalisation and ``F" means a floating normalisation uncertainty. Some of the systematic uncertainties are split into several components for a more accurate treatment. The relative systematic uncertainties in the inclusive expected SM background yields determined from the VLQ candidate invariant mass distribution after the fit to the background-only hypothesis are given in the last column in percentage. The tt̄ and W+jets background scaling-factor uncertainties (last two rows in the table) are the relative systematic uncertainties in the predicted tt̄ and W+jets background respectively.

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Table 04

Event yields in the SR and the tt̄ and W+jets CRs after the fit to the background-only hypothesis. The uncertainties include statistical and systematic uncertainties. Due to correlations among the SM backgrounds and the corresponding nuisance parameters, the uncertainties in the individual background components can be larger than the uncertainty in the sum of the background, which is strongly constrained by the data.

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Table 05

Observed and expected 95% CL upper limits on |sinθ_L| and c_L^Wb for a left-handed T quark in a T singlet model with masses of 800 GeV to 1200 GeV assuming B(T → Wb) = 0.5. The ±1σ and ±2σ uncertainties in the expected limits are also given.

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Table 06

Observed and expected 95% CL upper limits on |sinθ_R| and c_R^Wb for a right-handed Y quark in a (B,Y) doublet model with masses of 800 GeV to 1800 GeV. The ±1σ and ±2σ uncertainties in the expected limits are also given.

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Table 07

Observed and expected 95% CL upper limits on |sinθ_L| and c_L^Wb for a left-handed Y quark in a (T,B,Y) triplet model with masses of 800 GeV to 1600 GeV. The ±1σ and ±2σ uncertainties in the expected limits are also given.

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Table 08

Observed and expected 95% CL_s upper limits on the coupling value c_L^Wb for a left-handed Y signal with interference in a (T,B,Y) triplet model with masses of 800 GeV to 1600 GeV. The ±1σ and ±2σ uncertainties in the expected limits are also provided.

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Table 09

Observed and expected 95% CL_s upper limits on |sinθ_L| for a left-handed Y signal with interference in a (T,B,Y) triplet model with masses of 800 GeV to 1600 GeV. The ±1σ and ±2σ uncertainties in the expected limits are also provided.

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Table 10

Observed and expected 95% CL_s upper limits on the production cross-section times branching ratio (σ × B) for Q (=T or Y) signals with masses of 800 GeV to 2000 GeV for a no-interference case. The ±1σ and ±2σ uncertainties in the expected limits are also provided. The results are not physically meaningful and only presented to facilitate the comparison with an analysis in which the interference is not considered.

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Table 11

Signal cutflow for a generated Y signal with a mass of 900 GeV, 1200 GeV and 1600 GeV and a coupling strength of c_L^Wb = c_R^Wb ≈ 0.5 (√(c_L^Wb)² + (c_R^Wb)² ≈ 0.7) for the no-interference case. The number of signal events are calculated for the NLO cross section and an integrated luminosity of 36.1 fb^-1. Only statistical uncertainties are shown.

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Table 12

Signal acceptance times efficiency including the leptonic W branching fractions in percent for the Y signal in the SR. The uncertainty includes the statistical uncertainty in the Monte Carlo predictions.

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Auxiliary material

Figure 01

Comparison of data with SM predictions of the VLQ candidate mass distribution in the SR. The background prediction is shown pre-fit, i.e. before the fit to the data m_VLQ distributions in the control and signal regions under the background-only hypothesis. The uncertainty band includes the statistical uncertainty in the Monte Carlo predictions and the total systematic uncertainties added in quadrature. The first and last bin include the underflow and overflow respectively.

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Figure 02

Observed background yields in the SR and in the two CRs before the fit to the data in the control regions and the signal region under the background-only hypothesis (pre-fit). The lower panel shows the ratio of data to the background yields. The error bars represent the statistical uncertainty in the data. The uncertainty band includes the statistical uncertainty in the Monte Carlo predictions and the total systematic uncertainties added in quadrature.

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Figure 03a

(a) Comparison of data with SM predictions of the VLQ candidate mass distribution in the W+jets CR. (b) Comparison of data with SM predictions of the VLQ candidate mass distribution in the tt̄ CR. The background prediction is shown pre-fit, i.e. before the fit to the data m_VLQ distributions in the control and signal regions under the background-only hypothesis. The uncertainty band includes the statistical uncertainty in the Monte Carlo predictions and the total systematic uncertainties added in quadrature. The first and last bin include the underflow and overflow respectively.

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Figure 03b

Figure 04a

Distributions of (a) lepton p_T, (b) leading b-tagged jet p_T, and (c) E_T^miss; in the SR for data and the SM background processes with their post-fit normalisations. The uncertainty band includes statistical and all systematic uncertainties. The red dashed line shows a Y signal with a mass of 1200 GeV and a coupling of √(c_L^Wb)² + (c_R^Wb)² ≈ 0.5. The first and last bin include the underflow and overflow respectively.

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Figure 04b

Figure 04c

Figure 05

Observed (solid line) and expected (short-dashed line) 95% CL_s cross-section times branching ratio limits for the case of a VLQ Q (=T or Y) quark as a function of VLQ mass for a no-interference case. For the theoretical prediction, the branching ratio BR(Q → Wb) is set to one, as it is the case for Q=Y. The surrounding bands correspond to ±1 and ±2 standard deviations around the expected limit. The theoretical NLO cross sections for different coupling values are shown for the calculation using narrow-width approximation (dashed-dotted lines) and without using narrow-width approximation (solid lines). The results are not physically meaningful and only presented to facilitate the comparison with an analysis in which the interference is not considered.

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Figure 06a

(a) Comparison of invariant mass distributions in the SR of a 1.2 TeV Y signal with coupling c_L^Wb ≈ 0.71 and no interference with the SM background considered (dashed line) with a 1.2 TeV Y signal with left-handed only coupling of c_L^Wb ≈ 0.32 and with interference with the SM background considered (solid line). (b) Comparison of the shape of the SR invariant mass distributions of a 1.2 TeV Y signal with coupling c_L^Wb ≈ 0.71 and no interference with the SM background considered (dashed line) with a 1.2 TeV Y signal with right-handed only coupling of c_R^Wb ≈ 0.27 and interference with the SM background considered (solid line). The distributions for a right-handed and left-handed Y signal without considering any interference effect are the same.

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Figure 06b

Figure 07a

(a) Comparison of invariant mass distributions in the SR of a 1.2 TeV Y signal with coupling c_L^Wb ≈ 0.71 and no interference with the SM background considered (dashed line) with a 1.2 TeV Y signal with the coupling of c_L^Wb ≈ 0.32 and no interference with the SM background considered (solid line). The distributions for a right-handed and left-handed Y signal without considering any interference effect are the same. (b) Comparison of the shape of the SR invariant mass distributions of a 1.2 TeV T signal with coupling c_L^Wb ≈ 0.71 and no interference with the SM background considered (dashed line) with a 1.2 TeV T signal with left-handed only coupling of c_L^Wb ≈ 0.50 and interference with the SM background considered (solid line).

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Figure 07b

(a) Comparison of invariant mass distributions in the SR of a 1.2 TeV Y signal with coupling c_L^Wb ≈ 0.71 and no interference with the SM background considered (dashed line) with a 1.2 TeV Y signal with the coupling of c_L^Wb ≈ 0.32 and no interference with the SM background considered (solid line). The distributions for a right-handed and left-handed Y signal without considering any interference effect are the same. (b) Comparison of the shape of the SR invariant mass distributions of a 1.2 TeV T signal with coupling c_L^Wb ≈ 0.71 and no interference with the SM background considered (dashed line) with a 1.2 TeV T signal with left-handed only coupling of c_L^Wb ≈ 0.50 and interference with the SM background considered (solid line).

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Figure 08

Observed (solid line) and expected (short-dashed line) 95% CL cross-section times branching ratio limits for the case of the left-handed Y where interference was considered as a function of VLQ mass. The surrounding bands correspond to ±1 and ±2 standard deviations around the expected limit. The results are not physically meaningful and only presented to facilitate the comparison with an analysis in which the interference is not considered.

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Figure 09

Observed (solid line) and expected (short-dashed line) 95% CL cross-section times branching ratio limits for the case of the left-handed T where interference was considered as a function of VLQ mass. The surrounding bands correspond to ±1 and ±2 standard deviations around the expected limit. The results are not physically meaningful and only presented to facilitate the comparison with an analysis in which the interference is not considered.

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Figure 10

Observed (solid line) and expected (short-dashed line) 95% CL cross-section times branching ratio limits for the case of the right-handed Y (orange line) where interference was considered and for a VLQ Q quark (black line) as a function of VLQ mass for a no-interference case. The theoretical NLO cross sections for different coupling values are shown for the calculation using narrow-width approximation (dashed-dotted lines) and without using narrow-width approximation (solid lines). The results are not physically meaningful and only presented to facilitate the comparison with results of other analyses.

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Figure 11

Ranking of the nuisance parameters included in the fit according to their impact on the the measured signal strength, μ, for the right-handed Y signal of mass 1200 GeV with coupling of c_R^Wb ≈ 0.28 and considering interference of the signal with the SM background extracted from a fit on data with this 1200 GeV Y signal injected and fitted. Only the top 20 nuisance parameters are shown without including those related to MC statistical uncertainties. The empty rectangles correspond to the pre-fit impact on μ and the filled ones to the post-fit impact on μ, both referring to the upper scale. The impact of each nuisance parameter, Δμ, is computed by comparing the nominal best-fit value of μ with the result of the fit when fixing the considered nuisance parameter to its best-fit value, θ̂, shifted by its pre-fit (post-fit) uncertainties, ±Δθ (±Δθ̂). The black points show the pulls of the nuisance parameters relative to their nominal values, θ₀. These pulls and their relative post-fit errors, ±Δθ̂/Δθ, refer to the lower scale. The fitted signal strength μ for a signal-plus-background fit to data including both systematic and statistical uncertainties and only statistical uncertainties for the aforementioned signal is μ = -0.50 ± 0.70 and μ = -0.10 ± 0.42 respectively, which shows that the search is hence dominated by systematic uncertainties rather than statistical uncertainties.

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Figure 12

Ranking of the nuisance parameters included in the fit according to their impact on the the measured signal strength, μ, for the right-handed Y signal of mass 900 GeV with coupling of c_R^Wb ≈ 0.27 and considering interference of the signal with the SM background extracted from a fit on data with this 900 GeV Y signal injected and fitted. Only the top 20 nuisance parameters are shown without including those related to MC statistical uncertainties. The empty rectangles correspond to the pre-fit impact on μ and the filled ones to the post-fit impact on μ, both referring to the upper scale. The impact of each nuisance parameter, Δμ, is computed by comparing the nominal best-fit value of μ with the result of the fit when fixing the considered nuisance parameter to its best-fit value, θ̂, shifted by its pre-fit (post-fit) uncertainties, ±Δθ (±Δθ̂). The black points show the pulls of the nuisance parameters relative to their nominal values, θ₀. These pulls and their relative post-fit errors, ±Δθ̂/Δθ, refer to the lower scale. The fitted signal strength μ for a signal-plus-background fit to data including both systematic and statistical uncertainties and only statistical uncertainties for the aforementioned signal is μ = -0.11 ± 0.48 and μ = -0.12 ± 0.28 respectively, which shows that the search is hence dominated by systematic uncertainties rather than statistical uncertainties.

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Figure 13a

Distribution of the transverse momentum of the leading jet after applying preselection cuts and requiring that the leading jet is a b-tagged jet. The W+jets leading jet transverse momentum weights used for the W+jets leading jet p_T correction are determined from this distribution (Fig. (a)). The shaded error band depicts the statistical and systematic uncertainty in the SM prediction, taken as fully uncorrelated, before the reweighting; after the reweighting (Fig. (b)), the shaded error band depicts the statistical uncertainty in the SM prediction. The error attached to the data points is the corresponding statistical uncertainty.

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Figure 13b

Figure 14a

Distributions of the reconstructed VLQ mass after applying preselection cuts and requiring that the leading jet is a b-tagged jet for the SM background processes and data events (a) before and (b) after applying the W+jets leading jet p_T correction. The residual difference of about 10% between the data and the SM simulation in the tail of the invariant mass distribution of the reconstructed VLQ candidate after applying the W+jets leading jet p_T correction is considered in an extra systematic uncertainty. The shaded error band depicts the statistical and systematic uncertainty in the SM prediction without considering any W+jets correction systematic, taken as fully uncorrelated, before the re-weighting; after the re-weighting (Fig. 21(b)), the shaded error band depicts the statistical uncertainty in the SM prediction. The error attached to the data points is the corresponding statistical uncertainty.

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Figure 14b

Table 01

Observed and expected 95% CL_s upper limits on the production cross-section times branching ratio (σ × B) for the right-handed (RH) Y signal with interference with the SM background considered with masses from 800 GeV to 2000 GeV. The ±1σ and ±2σ uncertainties in the expected limits are also provided. The results are not physically meaningful and only presented to facilitate the comparison with an analysis in which the interference is not considered.

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Table 02

Observed and expected 95% CL_s upper limits on the production cross-section times branching ratio (σ × B) for the left-handed (LH) T signal with interference with the SM background considered with masses from 800 GeV to 1200 GeV. The ±1σ and ±2σ uncertainties in the expected limits are also provided. The results are not physically meaningful and only presented to facilitate the comparison with an analysis in which the interference is not considered.

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Table 03

Observed and expected 95% CL_s upper limits on the production cross-section times branching ratio (σ × B) for the left-handed (LH) Y signal with interference with the SM background considered with masses from 800 GeV to 1600 GeV. The ±1σ and ±2σ uncertainties in the expected limits are also provided. The results are not physically meaningful and only presented to facilitate the comparison with an analysis in which the interference is not considered.

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Table 04

Observed and expected 95% CL_s upper limits on the coupling values c_R^Wb for a right-handed Y signal with interference in a (B,Y) doublet model with masses of 800 GeV to 1800 GeV. The ±1σ and ±2σ uncertainties in the expected limits are also provided.

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Table 05

Observed and expected 95% CL_s upper limits on |sinθ_R| for a right-handed Y signal with interference in a (B,Y) doublet model with masses of 800 GeV to 1900 GeV. The ±1σ and ±2σ uncertainties in the expected limits are also provided.

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Table 06

Observed and expected 95% CL_s upper limits on the coupling c_L^Wb for a T signal with interference in a T singlet model with masses of 800 GeV to 1200 GeV. The ±1σ and ±2σ uncertainties in the expected limits are also provided.

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Table 07

Observed and expected 95% CL_s upper limits on |sinθ_L| for a T signal with interference in a T singlet model with masses of 800 GeV to 1200 GeV. The ±1σ and ±2σ uncertainties in the expected limits are also provided.

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Table 08

Table 09

Table 10

Table 11

Signal cutflow for a generated Y signal with a mass of 900 GeV, 1200 GeV and 1600 GeV and a coupling strength of c_L^Wb=c_R^Wb ≈ 0.5 (√(c_L^Wb)² + (c_R^Wb)² ≈ 0.7) for the no-interference case. The number of signal events are calculated for the NLO cross section and an integrated luminosity of 36.1 fb^-1. Only statistical uncertainties are shown.

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Table 12